Pre-existing Layer Research Articles

Sudden compression of the outer magnetosphere associated with an ionospheric mass ejection

On September 24, 1998 at 2345 UT the magnetosphere was suddenly compressed as the dynamic pressure of the solar wind rapidly rose from 2 to 15 nPa. At the Polar spacecraft, at high altitudes above the center of the northern polar cap, a remarkably smooth increase in the field strength occurred while the plasma properties changed abruptly, as described in an accompanying paper. Comparisons with models and an examination of the wave amplitudes during the compression indicate that the initial change in plasma properties was most probably due to convection of pre‐existing boundary layer plasma to the location of Polar rather than due to local heating by betatron acceleration and ion cyclotron waves. The smoothness of the increase in field strength is attributed to the very high velocity of compressional waves in the tail that outrun the advancing solar wind disturbance. The signatures as measured by GOES 10 at 1444 LT and at GOES 8 at 1846 LT in low latitude geosynchronous orbit are the more familiar sudden jump on the dayside, where the density is high and the compressional wave velocity low, and a weak change on the nightside, where tail current changes oppose the effects of the dayside magnetopause currents. This event is an ideal candidate for collaborative investigation of the effects of a classical sudden storm commencement on the magnetosphere.

Wide‐angle seismic constraints on the evolution of the deep San Andreas plate boundary by Mendocino triple junction migration

Recent, wide‐angle seismic observations that constrain the existence and structure of a mafic layer in the lower crust place strong constraints on the evolution of the San Andreas plate boundary system in northern and central California. Northward migration of the Mendocino Triple Junction and the subducted Juan de Fuca lithospheric slab creates a gap under the continent in the new strike‐slip system. This gap must be filled by either asthenospheric upwelling or a northward migrating slab attached to the Pacific plate. Both processes emplace a mafic layer, either magmatic underplating or oceanic crust, beneath the California Coast Ranges. A slab of oceanic lithosphere attached to the Pacific plate is inconsistent with the seismic observation that the strike‐slip faults cut through the mafic layer to the mantle, detaching the layer from the Pacific plate. The layer could only be attached to the Pacific plate if large vertical offsets and other complex structures observed beneath several strike‐slip faults are original oceanic structures that are not caused by the faults. Otherwise, if oceanic slabs exist beneath California, they do not migrate north to fill the growing slab gap. The extreme heat pulse created by asthenospheric upwelling is inconsistent with several constraints from the seismic data, including a shallower depth to the slab gap than is predicted by heat flow models, seismic velocity and structure that are inconsistent with melting or metamorphism of the overlying silicic crust, and a high seismic velocity in the upper mantle. Yet either the Pacific slab model or the asthenospheric upwelling model must be correct. While the mafic material in the lower crust could have been emplaced prior to triple junction migration, the deeper slab gap must still be filled. A preexisting mafic layer does not reduce the inconsistencies of the Pacific slab model. Such material could, however, compensate for the decrease in mafic magma that would be produced if asthenospheric upwelling occurred at a lower temperature. These low temperatures, however, may be inconsistent with asthenospheric rheology.

Strain localization mechanisms in deep-seated layered rocks

More than 160 shear zones of the Norwegian Caledonides and Italian Southern Alps, formed under high-temperature or high-pressure conditions, are analysed with respect to mechanisms of strain localization. In metabasic rocks with a pre-existing (magmatic) layering strain localizes preferentially in mafic layers, although experiments in monomineralic rocks would predict a location in the weaker feldspar-rich layers. In addition, in some of these zones amphibole recrystallizes dynamically, whereas feldspars only show undulatory extinction. These are evidences for strength inversion between feldspars and mafic minerals. This inverse strength behaviour is caused by a reduced grain size of feldspars and sometimes amphiboles in the pre-existing mafic layers, i.e. prior to shearing. The localization of the shear zones is therefore a grain-size-sensitive process (grain-size-dependent softening), nearly independent of the composition of the deformed material. The numerous amphibole–amphibole contacts favour dynamical recrystallization by grain-boundary migration and the rare feldspar–feldspar contacts prevent or suppress the same process in feldspars. Therefore, the feigned inverse strength is mainly caused by the interaction of reduced grain size and grain-boundary effects in the original rock. This behaviour can lead to the preferential localization of shear zones in mafic layers, although they are not favourably oriented with respect to the stress system.

Role of land surface in controlling daytime cloud amount: Two case studies in the GCIP‐SW area

Two summertime cases are studied, in which fields of low cumulus clouds develop out of an initially clear sky under quiescent synoptic scale conditions. Both sites fall within the southwest study area of the GEWEX (Global Energy and Water cycle Experiment) Continental‐Scale International Project (GCIP). The cases are modeled using a one‐dimensional soil‐vegetation‐boundary layer model known as the parameterization for land‐atmosphere‐cloud exchange (PLACE). PLACE accurately predicts cloud onset and amount in both cases, one from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) in Kansas and one from Oklahoma. Sensitivity tests with PLACE show further that in the Oklahoma case, morning cloud amount is highly sensitive to land surface characteristics, whereas in the FIFE case, much less sensitivity is found, primarily due to differences in the predawn atmospheric thermodynamic structure through which the boundary layer must grow. Analysis of observational data, including cloud amount inferred from GOES visible data and vegetation index from the advanced very high resolution radiometer (AVHRR) data confirm that the PLACE model predictions of sensitivities and of the sensitivity differences between cases are correct. In the Oklahoma case, atmospheric conditions and soil moisture are relatively uniform across the study area, but a significant gradient of satellite‐derived vegetation index is observed. Clouds form quickly due to a very shallow nocturnal inversion, with strong lapse conditions above. They form first over the hotter, more sparsely vegetated areas. Over areas covered with deciduous forest, clouds were observed to form 1 to 2 hours later due to the suppression of vertical mixing caused by increased latent heat release. In the FIFE case, because of a deep preexisting stable layer in the atmosphere the primary cause of spatial variations in morning cloud amount appears to be the amount of moisture in low levels of the atmosphere. In the afternoon, once the stable layer has been eroded, the FIFE study area begins to demonstrate more sensitivity of cloud amount to surface characteristics.

Abyssal Circulation in a Circumpolar Basin Driven by Discrete Sources of Buoyancy

Abstract The response of the deep ocean to periodic and steady forcing by mass sources is considered, in the presence of fluid loss, diffusion, and topography, which may or may not have regions of closed planetary vorticity. There can be a preexisting deep layer, or the forcing may produce one. The long-time response to forcing, which varies annually, is shown to have only a small periodic component, essentially because the mass loss by diapycnic transfer is weak. This is in qualitative agreement with observations of overflows, which show little seasonal signal. Ridges do not form an effective block to the flow, which can bypass the ridges, approximately following lines of constant planetary vorticity. A brief discussion of how fluid leaks out of closed planetary vorticity regions by diffusion is included.

Evidence for extension in the western Alpine orogen: the contact between the oceanic Piemonte and overlying continental Sesia units

In the Alps and other orogens, major tectonic contacts must have been established during convergence and overthrusting. What is less clear is how strongly these contacts were modified as the orogen evolved. During Alpine collision, continental crust of the overriding plate, the Sesia unit, is inferred to have been thrust northwest over oceanic material of the Piemonte unit. Both units preserve, in parts, eclogite facies metamorphism. However, all structures along this contact in our study area indicate that final movement was southeast directed (with no clear evidence of the kinematics of earlier movement). These structures include SE-verging folds, SE-directed shear bands, and larger normal faults downthrowing to the southeast. It is inferred that there was a SE-directed shear regime below the contact which, although locally buckling and shortening earlier lithological contacts, was dominantly extensional with respect to pre-existing tectonic layering. The lower limit of this shear regime has not been identified, although SE-directed shear is common throughout the upper, greenschist facies part of the Piemonte unit. We show that this shear passes beneath the Sesia zone and does not re-emerge. It is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface. It may have contributed to the unroofing of eclogite facies rocks in the lower part of the Piemonte unit, although additional timing data are required to clarify this. Nearby, SE-directed shears buckle and imbricate earlier layering within the Piemonte unit. These are normally identified as backthrusts, yet they do not appear to breach the base of the Sesia unit and might merge with the extensional shear. Even though these structures shorten layering, they too could have been extensional relative to the Earth's surface. The significance of Alpine ‘backthrusts’ should be reappraised in this context. This study indicates that hinterland-directed extension could have been an important phenomenon during Alpine evolution.

Buoyancy-driven Abyssal Circulation in a Circumpolar Ocean

Abstract Simple models are developed to describe the abyssal circulation in a circumpolar ocean driven by localized annual sources of water representing convection events. Models are based on a geostrophic reduced-gravity formulation and are located on a zonally periodic beta plane. Nonlinear analytical solutions are first obtained for the evolution of an initially prescribed water mass distribution in the presence of a zonal mean flow and topography that is a function of the meridional coordinate only. For large time, our model suggests that diffusion results in a zonally uniform interface height with associated geostrophic currents if the initial water mass has nontrivial zonally averaged meridional structure. Possible influence of neglected effects on this large time behavior are discussed. Solutions are also presented for the evolution of an abyssal water mass introduced into an area where no equally dense water existed previously, and for a water mass added to a preexisting abyssal layer of equal den...

Deformation Mechanisms in a Laves Phase

The stress-induced phase transformation between C36 and C15 structures in Fe2Zr is studied by electron microscopy. The nucleus of the transformation is believed to be some pre-existing C15 layers in C36 particles. Microstructural evidence for three mechanisms of growth of a new phase were found: Fault accumulation and rearrangement, moving of individual partial dislocations between two phases, and the migration of microscopic ledges composed of a series of Shockley partials between C36 and C15. Plastic deformation by slip on non-basal planes of C36 caused by indentation is studied.

Gravity wave generation of simultaneous auroral sporadic- E layers and sudden neutral sodium layers

Measurements from the EISCAT incoherent scatter radar are used to show that the action of a gravity wave, generated by auroral Joule heating, could be the mechanism responsible for the simultaneous occurrence of sporadic- E layers and sudden neutral sodium layers. The radar measurements were obtained during an event previously reported on the basis of ionosonde and lidar observations ( Von Zahn and Hansen, 1988, J. atmos. terr. Phys. 50, 93). The EISCAT observations indicate that downward neutral air motions, probably associated with gravity waves, correlate with the appearance of sporadic- E layers. Computer simulations are used to demonstrate that the observed vertical winds can produce thin layers in a pre-existing broad layer of minor constituents, such as metallic atoms or ions, resulting in sudden or sporadic- E layers, respectively.

The effect of gaseous atmosphere and pre-treatment on the oxidation-nitridation of Fe-31 Mn-9Al-6Cr-0.86C alloy at 1000� C

The Fe-Mn-Al-based alloys have attracted considerable interest for their potential application in stainless steel usage. In this study the oxidation and nitridation behaviour of Fe-31 Mn-9Al-6Cr-0.86C (wt%) alloy is investigated. The effects of a gaseous atmosphere, a pre-existing oxide layer and the alloy phase distribution in the matrix are evaluated with respect to the morphological development and kinetics. The nitridation behaviour of the alloy in air is sensitive to the alloy surface condition, such as the alloy phases in the matrix and the oxides in the scale. For the aged multiphase alloy, oxidation occurs in the ferrite phase and nitridation takes place in the austenite phase due to the higher solubility of nitrogen. The existing interphase boundaries provide an easy path for nitrogen migration and thus promote the nitridation. However, the surface oxide layers act as a diffusion barrier for nitrogen. The effectiveness of the barrier in the retardation of nitridation depends on the amounts of MnAl2O4 and α-Al2O3 in the oxide layer.

Ion Beam Processing of GaAs at Elevated Temperatures

ABSTRACTElevated temperature ion bombardment of GaAs has been examined to investigate the nature of residual damage and the interplay between bombardment-induced defect production and dynamic annealing. The nature of disorder is found to depend strongly on ion energy, species, dose, dose rate and substrate temperature. A temperature regime is identified in which dynamic annealing leads both to the efficient formation of band gap traps for carrier removal and to the low temperature crystallization of pre-existing amorphous layers.

Morphogenetic restructuring and formation of basement membranes by Sertoli cells and testis peritubular cells in co-culture: Inhibition of the morphogenetic cascade by cyclic AMP derivatives and by blocking direct cell contact

Addition of dibutyryl cyclic AMP (dbcAMP), methylisobutylxanthine (MIX), or cytochalasin D to co-cultures of Sertoli cells and testicular peritubular myoid cells blocks a series of morphogenetic changes which otherwise occur during culture. When Sertoli cells are plated directly onto preexisting layers of peritubular cells maintained under basal conditions, structures form which display many of the characteristics of germ cell-depleted seminiferous tubules. The presence of dbcAMP, MIX, or cytochalasin D, added at varying times after plating Sertoli cells, results in the inhibition of each successive stage of in vitro remodeling: the inhibition of migration of Sertoli cells, the inhibition of initial ridge formation, the blockage of subsequent formation of mounds and nodules of compacted Sertoli cell aggregates, the prevention of the formation of basal lamina and associated layers of extracellular matrix between Sertoli cell aggregates and surrounding peritubular cells, and the inhibition of tubule formation. The presence of dbcAMP also inhibits the migration of peritubular cells, contractions by these cells, and compaction of Sertoli cell aggregates. When intimate cell apposition is prevented by plating the two cell types on either side of a membrane filter, the morphogenetic cascade is blocked, and no formation of a germ cell-depleted seminiferous tubule-like structure occurs. Other effects of dbcAMP on cell shape, cell movement, and cell association patterns during co-culture are described. Possible mechanisms by which dbcAMP, MIX, or cytochalasin D blocks restructuring are discussed. Since each elicits perturbations of the cytoskeleton, we offer the interpretation that cytoskeletal changes may be correlated with the prevention of closely apposing cell compact and the inhibition of basement membrane formation. Interactions observed between Sertoli cells and peritubular cells during co-culture are postulated to be analogous to those occurring in other types of mesenchymal cell-epithelial cell interactions during organogenesis and during tubulogenesis in the fetal testis. Speculatively, the blockage by dbcAMP of the morphogenetic cascade in the co-cultured system may be related to the inhibition by dbcAMP of testis cord formation in organ cultures of fetal gonads reported by others.

Detailed observations of the plasma sheet during a substorm on April 24, 1979

This paper reports a detailed study of plasma, magnetic field, and energetic particle data acquired by the ISEE 1 and 2 satellites, about 20 RE from earth in the magnetotail plasma sheet during a well‐defined substorm on April 24, 1979. The paper addresses, particularly, the plasma flows and energetic particle anisotropies that were encountered during the various stages of the substorm's evolution and compares these to predictions of the neutral line model of substorms. We find that the observations support that model in considerable detail. The interval from substorm onset at earth to plasma dropout was only 4½ min, and we stress the importance of high time resolution in all measurements if a full understanding of substorm initiation is to be developed. We found that the energetic ions (up to 125 keV) and plasma ions, although they sometimes showed somewhat different angular distributions in the plasma rest frame, typically shared a common bulk speed, i.e., the bulk flow speed of the plasma, parallel and perpendicular to the magnetic field. Thus the anisotropies of the energetic ion fluxes that have sometimes led to their characterization as earthward or tailward “beams” were found to be largely a consequence of their bulk motion with the plasma. We identify the region at the lobe‐plasma sheet interface, which has been called the plasma sheet boundary layer, with the separatrix layer in the neutral line model. This is the region of field lines including, and just equatorward of, the lobe‐plasma sheet separatrix, which should carry the most recent dynamic signatures of reconnection. Our results seem to support that expectation. Several features of our observations were inconsistent with the substorm picture, called the boundary layer dynamics model, in which a satellite's observation of fast plasma and particle flow is due primarily to its envelopment by a preexisting boundary layer of flowing plasma.

The Competition between Ion Beam Induced Epitaxial Crystallization and Amorphization in Silicon: The Role of the Divacancy

AbstractThe transition from ion induced epitaxial crystallization to planar amorphization of a preexisting amorphous layer in silicon has been investigated. The conditions for dynamic equilibrium at the transition were determined for different ion species as a function of dose rate and temperature. The critical dose rate for equilibrium varies exponentially with 1/T, exhibiting an activation energy of ∼1.2 eV. Furthermore, for different ions, the critical dose rate is inversely proportional to the square of the linear displacement density created by individual ions. This second order defect production process and the activation energy, which is characteristic of divacancy dissociation, suggest that the accumulation of divacancies at the amorphous/crystalline interface controls the balance between crystallization and amorphization.

Ductal spread in prostatic carcinoma

The histologic appearances of ductal invasion were studied in 139 cases of prostatic adenocarcinoma diagnosed in the Department of Pathology, Howard University Hospital, during the period January 1980 through October 1983. Intraductal spread was found in almost half (48%) of the prostatic glands examined. Ductal spread was associated with the local extent (P less than 0.001) rather than with the grade of the tumor (P less than 0.01). Three distinct patterns of ductal penetration were recognized. The duct wall was completely destroyed in microinvasion. In foci of ductal permeation the integrity of the basement membrane was generally preserved, and the duct wall was infiltrated mainly by solitary tumor cells. When the tumor spread was by extension in continuity within the duct wall, the neoplastic cells appeared to grow between the pre-existing epithelial layers. It was concluded that prostatic carcinoma cells have the ability to penetrate the wall of benign ducts and progressively replace the normal epithelial elements. In this process the general framework of the affected duct appears to be preserved.

The mechanism of electrodeposition of cadmium sulphide on inert metals from dimethylsulphoxide solution

The mechanism, of electrodeposition of CdS on Hg, and Pt electrodes has been studied in dimethylsulphoxide solutions by performing cyclic voltammetry at stationary electrodes and pulse polarography at dropping Hg electrodes in solutions containing both Cd(II) ions and elemental sulphur. Evidence is presented to demonstrate that sulphur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulphide is probable. As a result, formation of the initial layer of CdS involves reaction of Cd(II) ions with chemisorbed sulphur or with a pre-existing layer of metal sulphide. Data for the formation of CdS are compared with those obtained for the formation of PbS and ZnS.

The mechanism of electrodeposition of cadmium sulphide on inert metals from dimethylsulphoxide solution

The mechanism, of electrodeposition of CdS on Hg, and Pt electrodes has been studied in dimethylsulphoxide solutions by performing cyclic voltammetry at stationary electrodes and pulse polarography at dropping Hg electrodes in solutions containing both Cd(II) ions and elemental sulphur. Evidence is presented to demonstrate that sulphur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulphide is probable. As a result, formation of the initial layer of CdS involves reaction of Cd(II) ions with chemisorbed sulphur or with a pre-existing layer of metal sulphide. Data for the formation of CdS are compared with those obtained for the formation of PbS and ZnS.

Layered convection and crystal layers in multicomponent systems

Laboratory experiments1–5 in aqueous solutions have been used to model some of the physical and dynamical processes occurring during crystallization in magma chambers. In particular, it has been demonstrated2,3,5 that crystallization at the side walls can lead to density stratification and layering in an initially homogeneous container, and also to the formation of vertical compositional gradients (that is, to ‘differentiation’) when more than one solute is present. Density gradients and compositional layers can alternatively be set up by replenishing a container from below with fluid of different temperature and composition4,6,7. Underlying all this work has been the concept that some aspects of the regularity and large horizontal extent of layering in well-documented igneous intrusions8,9 might be explained in terms of ‘double-diffusive’ effects in the fluid state, with the growth of crystals subsequently being affected by these pre-existing layers. In the latest experiments reported here, we have explicitly addressed the problems of horizontal layering produced in the crystals themselves, when a compositionally-stratified tank of fluid containing several solutes has been cooled from either below or above. The relationship between the scales of the layers in the fluid and in the crystals has also been examined.

The damping of structural vibration by thin gas films

A theoretical and experimental investigation of the processes by which thin layers of gas trapped between surfaces of a structure may contribute to damping of structural vibrations is described. Effects of gas compressibility, inertia and thermal conductivity are allowed for in the theory. Particular attention is given to the variation of damping with gas pressure and layer thickness. The aim of the investigation was to provide results of use in the design of vibration absorbers for use in pressurized gas environments, as well as to contribute to a better understanding of the behaviour of pre-existing gas layers in structures.

Crystallization in a double‐diffusive system

The experiments reported here are a first attempt to model in a simple laboratory system some of the physical processes occurring during crystallization in magma chambers and to examine possible effects of different molecular diffusion rates on the formation of layered igneous rocks. Our laboratory results suggest that stratification in the melt and ‘double‐diffusive’effects during solidification of the magma should be taken into account when developing theories of the layering. Explicitly, we have extended earlier experiments on double‐diffusive convection, which show how layers may readily form from smooth gradients when two components with different molecular diffusivities have opposing effects on the density gradient. The new experiments included the influence of crystallization and have used an aqueous solution Of Na2CO3 cooled in various configurations. When cooling was from the top, the crystals formed in horizontal layers, whether the initial fluid was homogeneous, was stratified with a constant gradient, or was stratified in several distinct layers. The dominant effect was the formation of a cold but light fluid layer against the cooled upper boundary as the denser crystals grew. The crystals were small and closely packed in the upper layer (which was vigorously converting) and larger and more loosely packed in the lower, more quiescent fluid. In the case where there were preexisting layers there was an abrupt increase in the growth rate of crystals as they crossed an interface. With sidewall cooling and a constant gradient the lighter solution left behind after crystallization streamed upward in a thin boundary layer right to the top of the tank. The outer flow consisted of circulation in nearly horizontal converting layers, each of which was depressed slowly as the light fluid collected at the top. When a constant gradient of Na2CO3 was cooled from below, crystallization produced a ‘finger’ instability, leading to the growth of a mixed layer in the fluid above. An experiment using opposing gradients of Na2CO3 and K2CO3 (with Na2CO3 rich on top) showed that crystals settling to their own density level and then redissolving can reverse the original sense of the gradient and produce small‐scale double‐diffusive layering in the liquid.